feat(client): implement DSCP negotiation and socket traffic class application

Apply the negotiated DSCP value to the UDP socket after a successful session open and clear it before closing to support network-level quality of service. This change adds platform-specific socket option handling for IPv4 TOS and IPv6 Traffic Class.

crates/irtt-client/src/client/tests/mod.rs

@@ -11,6 +11,7 @@ use support::*;
 
 mod close;
 mod config;
+mod dscp;
 mod hmac;
 mod negotiation;
 mod no_test;